Indonesia's victims of Mount Merapi were buried in a mass grave after authorities gave relatives three days to identify bodies. Photograph: Adi Weda/EPA

The bodies of dozens of people killed by Mount Merapi, some of them too charred ever to be identified, were yesterday buried in a mass grave as the nearby volcano continued to fire hot ash and gases in the atmosphere.

As some relatives wept and others recited prayers, villagers and policemen unloaded the corpses from ambulances. They were placed into a large trench dug in a field in the shadow of the volcano, which has claimed 138 lives in the past two weeks.

Mount Merapi unleashed its most powerful eruption in a century last Friday, sending clouds of hot gas, rocks and debris sliding down its slopes at motorway speeds, smothering villages and leaving a trail of dead.

Islam requires that the dead be buried quickly, so authorities gave relatives three days to identify their loved ones. About 30 bodies were taken by relatives to be buried elsewhere.

Merapi showed no signs of abating yesterday. Explosions were heard as it shot ash up to four miles into the air, covering windscreens and rooftops hundreds of miles away. The ash hung so thickly in the air that breathing became painful.

The Indonesian government has put Yogyakarta, a city of 400,000 people only 20 miles from Merapi, on alert. Though there have been no orders to evacuate, many residents have fled.